States of MatterCambridge IGCSE Physics: Revision notes
Section 1
What are the three states of matter?
Matter exists in three distinct states: solid, liquid, and gas. Each state has unique properties determined by how closely packed the particles are and how much kinetic energy they possess. The state of a substance depends on temperature and pressure conditions.
The three states can be arranged in order of increasing particle energy and decreasing particle density:
- Solids – particles tightly packed, lowest energy
- Liquids – particles close together but with some freedom to move, intermediate energy
- Gases – particles far apart, highest energy
Think of particles in solids as dancers in a strict formation (fixed positions), liquids as people in a crowded room (moving but constrained), and gases as birds in the sky (spread out and moving freely in all directions).
Section 2
What are the distinguishing properties of solids?
Solids have a number of key distinguishing properties:
| Property | Description |
|---|---|
| Shape | Fixed and definite shape |
| Volume | Fixed and definite volume |
| Density | High density (particles closely packed) |
| Particle arrangement | Particles held in fixed, rigid positions |
| Particle motion | Particles vibrate about fixed positions only |
| Compressibility | Incompressible – particles cannot be forced closer together |
| Flow | Cannot flow |
The fixed shape and volume occur because particles are held in place by strong intermolecular forces. Solids are typically hard and rigid because of the rigid arrangement of particles.
Examiners often ask you to compare states by their shape and volume – always state both properties clearly. For solids, the key phrase is 'fixed shape AND fixed volume'.
Section 3
What are the distinguishing properties of liquids?
Liquids have distinctive properties that differ from both solids and gases:
| Property | Description |
|---|---|
| Shape | No fixed shape – takes the shape of its container |
| Volume | Fixed and definite volume |
| Density | High density (particles close together, but less than solids) |
| Particle arrangement | Particles close together but in random arrangement |
| Particle motion | Particles move freely and randomly in all directions |
| Compressibility | Incompressible – particles cannot be forced closer together |
| Flow | Can flow and take the shape of the container |
The key distinction between liquids and solids is that liquids have no fixed shape – they conform to the shape of whatever container they are placed in. However, they retain a fixed volume, which distinguishes them from gases.
Students often say liquids have 'no volume' when they mean 'no fixed shape'. Liquids always have a fixed volume – this is essential. If a liquid has no defined volume, it would be a gas.
Section 4
What are the distinguishing properties of gases?
Gases have characteristics that make them distinctly different from solids and liquids:
| Property | Description |
|---|---|
| Shape | No fixed shape – fills entire container |
| Volume | No fixed volume – expands to fill the container |
| Density | Very low density (particles far apart) |
| Particle arrangement | Particles widely spaced in random arrangement |
| Particle motion | Particles move rapidly and randomly in all directions |
| Compressibility | Highly compressible – particles can be forced closer together |
| Flow | Can flow easily |
Gases have the highest kinetic energy of the three states. The particles are so far apart that intermolecular forces are negligible. This explains why gases are the only state with no fixed volume – they will always expand to fill whatever space is available.
When comparing gases to other states, emphasise that gases have BOTH no fixed shape AND no fixed volume. This dual property is the defining feature that separates gases from solids and liquids.
Section 5
What are the terms for changes of state?
Matter can change from one state to another. Each transition has a specific scientific term:
| Change of State | Direction | Description |
|---|---|---|
| Melting | Solid → Liquid | Heat energy causes solid particles to vibrate so vigorously that they break free from fixed positions |
| Freezing | Liquid → Solid | Cooling removes kinetic energy; particles slow down and form a rigid structure |
| Evaporation | Liquid → Gas | Heat energy allows some particles to escape from the liquid surface and become a gas |
| Condensation | Gas → Liquid | Cooling removes kinetic energy; gas particles lose energy and form a liquid |
| Sublimation | Solid → Gas | Heat energy allows solid particles to change directly into a gas without becoming a liquid first |
| Deposition | Gas → Solid | Cooling allows gas particles to change directly into a solid without becoming a liquid first |
Key points:
- Melting and freezing occur at the melting point of a substance
- Evaporation and condensation involve heat energy transfer
- Sublimation and deposition are less common but must be known
- These changes are reversible – a substance can change states and return to its original state
Ice at 0 °C melts to water at 0 °C when heat is supplied. The temperature does not change during melting – the energy breaks intermolecular bonds rather than increasing particle kinetic energy. This is why 'melting point' is a single temperature, not a range.
Examiners test the correct terminology rigorously. Use 'evaporation' (not 'boiling' unless specifically asked) for liquid-to-gas changes, and always spell sublimation and deposition correctly – these less common terms are favourite exam questions.
Must Know
- Solids have fixed shape and fixed volume; particles are held in fixed positions and can only vibrate
- Liquids have no fixed shape (take the shape of the container) but fixed volume; particles move freely but remain close together
- Gases have no fixed shape and no fixed volume (fill the container); particles are far apart and move randomly with high kinetic energy
- Six changes of state must be known: melting (solid→liquid), freezing (liquid→solid), evaporation (liquid→gas), condensation (gas→liquid), sublimation (solid→gas), and deposition (gas→solid)
- All changes of state are reversible and involve transfer of heat energy – removing energy reverses the change
- Melting and freezing occur at the same temperature (melting point); compressibility increases dramatically from solids to gases
That's the notes covered.
Carry on to the next subtopic.